1. Field of the Invention
The present invention relates to methods for manufacturing fine particles and to apparatuses for manufacturing fine particles. More specifically, the present invention relates to a vapor-phase growth method for fine particles in nanometer sizes and to a manufacturing method thereof.
2. Description of the Related Art
Fine particles in nanometer sizes exert unprecedented functions due to a quantum size effect. Accordingly, fine particles are recently drawing attention as new substances. Depending on types of materials, such fine particles are applied to fluorescent materials for visible light LED elements and displays, magnetic recording media and the like.
In general, fine particles are fabricated by use of a vapor-phase growth method. FIG. 1 is a view showing a constitution of a conventional apparatus for manufacturing fine particles. For example, ZnS fine particles being fluorescent materials are manufactured in accordance with the following method by use of the apparatus shown in FIG. 1 (Okuyama et al. J. Materials Science, vol. 32, 1229-1237 (1997)).
Source gas containing Zn(NO3)2 and SC(NH2)2 is introduced into a reactor 101, in which it is adjusted to a normal pressure inert gas atmosphere, and heated up to a range from 600° C. to 700° C. with a heater 102 provided on the reactor 101. The source gas heated causes a chemical reaction as defined in the following formula (FI), thus ZnS fine particle cores are generated.Zn(NO3)2+SC(NH2)2→ZnS+2NO+CO2+N2+2H2O  (FI)
The ZnS fine particle cores grow larger in the process of moving inside the reactor. The ZnS fine particles thus obtained are discharged from the reactor 101 together with other gas and diluted with inert gas on the way, then the ZnS fine particles and the inert gas are introduced to a cooling device 103 and cooled down to a room temperature.
The cooled gas containing the generated fine particles passes through a collector 104 filled with a solution containing a surfactant. Thus, only the generated particles are collected in the solution and preserved in a dispersed state.
FIG. 2A and FIG. 2B are graphs concerning the above-described vapor-phase growth method, which show variations in the number of generation and sizes of fine particles with respect to reactive time starting from the time when the source gas is introduced into the reactor 101 and a fine particle generating reaction is initiated.
As shown in FIG. 2A, the number of generation of the fine particles increases constantly in the beginning with passage of time. However, the number of generation is saturated after 0.001 second or thereabout, and then the number of generation gradually decreases thereafter with time. After a lapse of 0.1 second, the degree of decrease in the number of generation of the fine particles is significantly accelerated.
In the meantime, as shown in FIG. 2B, an average particle size scarcely changes for 0.001 second or thereabout when the number of generation of the fine particles is constantly increasing. However, the average grain size starts increasing with passage of time after 0.001 second or thereabout when the number of generation of the fine particles is saturated.
Based on these data, it is conceivable that growth of the fine particles proceeds with the following three stages (I to III) according to the particle growth method using the conventional vapor-phase growth method.
First stage (I): The source gas is decomposed and the fine particle cores (molecules) are generated as seeds of particle generation. In this process, although the number of the fine particles increases, the fine particle sizes scarcely change (A fine particle core generating process).
Second stage (II): The generated fine particle cores are bonded together in a range from several to several hundred particles and grow into clusters in nanometer sizes. Therefore, at this stage, the number of generation of the fine particles decreases with passage of time; meanwhile, the fine particle sizes start increasing (A fine particle cluster generating process).
Third stage (III): The clusters generated in nanometer sizes cohere to form fine particles in sizes of 10 nanometers or larger. Accordingly, the number of generation of the fine particles decreases further (A cluster cohering process).
Among the above-described three stages, generation of the fine particle cores in sizes of 10 nanometers or below suitable for exerting a quantum effect progresses most efficiently during the first stage. However, as shown in FIG. 2A and FIG. 2B, the first stage ends in an extremely short period as the first stage lasts only for 0.001 second at the longest from initiation of the fine particle generating reaction. When the conventional vapor-phase growth method is applied, it is impossible to control the reactive time in a range within 0.1 second. Accordingly, the growth of the fine particles progresses toward the third stage inevitably. As a result, as shown in FIG. 3, the obtained fine particles include a considerable number of fine particles larger than 10 nanometers.
Therefore, in order to obtain the fine particles in the sizes within 10 nanometers which exert the quantum effect, required is an additional operation for extracting only the fine particles in a predetermined size range by use of a classifier out of the fine particles collected and preserved in accordance with the conventional manufacturing method. As a result, additional costs are involved upon manufacturing the fine particles.